Laser Induced Damage ofGrazing Incidence Metal Mirrors

Mark S. TillackT. K. Mau

Mofreh Zaghloul

High Average Power Laser Program WorkshopNov. 13-14, 2001Livermore, CA

Statement of purposeOur research seeks to develop improved understanding of damage mechanisms

and to demonstrate acceptable performance of grazing incidence metal mirrors, with an emphasis on the most critical concerns for laser fusion. Through both experimen-tation and modeling we will demonstrate the limitations on the operation of reflective optics for IFE chambers under prototypical environmental conditions.

Deliverables:

Measure LIDT at grazing incidence with smooth surfaces. Sept. 1, 2001

Model reflectivity and wavefront changes of smooth surfaces. Sept. 1, 2001

Measure effects of defects and surface contaminants on reflectivity, LIDT and wavefront. April 1, 2002

Model reflectivity and wavefront changes due to defects and April 1, 2002contamination.

Statement of Purpose and Deliverables

Accomplishments for this period of performance

Compare damage on 99.999% Al with Al-1100 done

Perform tests at 5 J/cm2 done

Perform sub-threshold irradiation of amorphous Al ongoingto explore recrystallization

Establish methods for creating contaminated surfaces done

Obtain samples for neutron irradiation • multi-layer dielectric mirror done

• Al mirror deferred

Exercise ZEMAX to assess wavefront degradation delayed

StatusGoal

Testing up to 180 J/cm2 has been completed by focusing the beam

fluence

• Damage occurs at a higher fluence compared with normal incidence

• Silicide occlusions in Al 6061 preferentially absorb light, causing explosive ejection and melting

• Fe impurities appear unaffected

Several shots at 80˚, 1 J/cm2 peak

Damage to Al 6061 at grazing angle (review)

Fe

Fe MgSi

1000x

Damage Regimes for Al-1100 (0.1% Cu, Fe)

1 J/cm2

4kX

1kX

20 J/cm2

Damage Mechanisms for Al-1100

Damage Estimates for Melting and Yielding

1. Estimate of energy required to melt:

T - To = (2q”/k) sqrt(tt/)e = q”t/[(1-R) cos]

T-To = 640˚Ct = 10 ns, =85˚e = 143 J/cm2

2. Estimate of energy required to cause plastic deformation:

y = E To/(1–) fully-constrainedE = 75 GPa, = 25x10–6 y = 13-24 ksi (100-150 MPa) T ~ 35˚Ce ~ 8-10 J/cm2

Damage Regimes for 99.999% pure Al

130X

180 J/cm2

104 shots

4 Distinct Regions Are Observed at 104 Shots

I

II IIIIV

Region I Region IV

Nikon EpiPhot

The Zone Adjacent to the Melted Area Exhibits Effects of Stress

Why Does the Surface Survive Above 10 J/cm2?

• Something about grazing incidence irradiation stabilizes the mechanical instability?

• Al2O3 capping layer stabilizes the interface?

What we plan to do to explain this:

• Perform additional studies on diamond-turned Al in the range of 10–50 J/cm2.

• Implement beam smoothing to help interpret the results

Methods of Controlled Surface Contamination

• Many material optionsoxides, metals, carbides, carbon10 nm – 1 m size

• Many vendorsNanoscale Materials Inc. (Nantek)Nanophase Technolgies Corp.Reade Advanced MaterialsAltair Technologies

• Cheap~$1/g

(1) Nanopowders

Methods of Controlled Surface Contamination

• AerosolUltrasonic atomizersMean droplet size as low as 2–10 mMisonix Inc.*, Sonics and Materials

• Condensed vaporNonvolatile liquid residueUniform coating using spray atomizer

(2) Aerosol and Condensibles

*$1300

Specularly reflected intensity is degradedby induced mirror surface roughness (review)

Isc=I0e−g+Id

e.g., at 1 = 80o, = 0.1, e-g = 0.97

• The effect of induced surface roughness on beam quality was investigated by Kirchhoff wave scattering theory.

• For cumulative laser-induced and thermomechanical damages, we assume Gaussian surface height statistics with rms height .

0 0.1 0.2 0.3 0.4 0.5

1.0

0.8

0.6

0.4

0.2

0

1 = 80o

70o

60o

Inte

nsi

t y D

e gr a

da t

ion

, e–g

• Grazing incidence is less affected by surface roughness

• To avoid loss of laser beam intensity, < 0.01

12

Isc

Io : reflected intensity from smooth surfaceId : scattered incoherent intensityg : (4 cos1/)2

Iinc

Laser Scattering from Anisotropic Defects

• Laser induced mirror damages can be spatially anisotropic and may have multiple scale lengths. The surface height statistics may be non-Gaussian.

• We plan to use Kirchhoff theory to determine the loss of specularly reflected intensity due to realistic mirror defects, and compare with experimental measurements.

• The total scattered scalar field is evaluated by the integral expression

where SM is the mirror surface, h(xo,yo) is the measured height distribution, F, A, B, and C are functions of the surface reflectivity, and the incident and reflected angles from which intensity profile distortion can be deduced. • Kirchhoff theory in vector form can be used to include induced depolarization effects on mirror reflectivity.

ψsc(r r ) =−ikeikr

4πrF exp[ik(Axo

SM

∫ +Byo +Ch(xo,yo)]dxodyo

Goals for next period of performance

• Try to understand/explain why pure Al survives beyond 10 J/cm2

• Perform tests with contaminated surfaces, acquire damage curves

• Perform tests on Al coated surfaces– Damage limits vs. coating technique and degree of attachment– Sub-threshold irradiation of amorphous Al

• Plan testing of MER mirrors

• Ray tracing analysis to determine deformation limits

• Kirchhoff analysis of correlated defects

Normal incidence reflectivity of several metals